The present invention relates generally to activatable and deactivatable security tags, of the type used with an electronic surveillance system for detecting the unauthorized removable of articles and, more particularly, two such security tags which include enhanced electrostatic protection.
The use of electronic article surveillance or security (EAS) systems for detecting and preventing theft or unauthorized removal of articles or goods from retail establishments and/or other facilities, such as libraries, has become widespread. In general, radio frequency type EAS systems utilize a label or security tag containing an electronic circuit such as an inductor/capacitor resonant circuit, which is secured to an article or the packaging for an article to be protected. A transmitter tuned to the frequency of the resonant circuit of the security tag (the detection frequency) is employed for transmitting electromagnetic energy into a surveillance or detection zone typically located proximate to the exit of a retail establishment or other facility. A receiver, also tuned to the resonant frequency of the security tag, is also located proximate to the surveillance zone. If an article containing an active security tag enters the detection zone, the resonant circuit of the tag resonates, establishing a disturbance in the electromagnetic field which is detected by the receiver to activate an alarm for alerting security personnel.
In order to prevent accidental activation of an alarm by a person who has actually purchased an article having a security tag or a person who is authorized to remove from a facility an article having a security tag, security tags must be deactivatable. One method for deactivating a security tag involves momentarily placing the tag near a deactivation device which subjects the tag to electromagnetic energy at the resonant frequency of the tag and at a power level sufficient to cause the resonant circuit to short circuit and, therefore not resonate at the detection frequency. In order to avoid having the deactivation electromagnetic energy at a high power level, deactivatable security tags typically have a deactivation feature, such as one or more capacitor elements in which the dielectric between, at least a portion of the plates of the capacitor elements is weakened or reduced so that the capacitor plates can be short circuited when exposed to electromagnetic energy at the resonant frequency at relatively low power levels. Other, more recently developed security tags are both activatable and deactivatable. Activatable/deactivatable security tags typically have a resonant circuit having at lease two capacitors, each of which includes a weakened or reduced dielectric area between the capacitor plates to facilitate short circuiting of the capacitors. The resonant circuit of an activatable/deactivatable tag typically has an initial resonant frequency, which is generally outside of the frequency range of the EAS system with which the tag is to be used. When the tag is exposed to a sufficient level of electromagnetic energy at the initial resonant frequency, one of the capacitors becomes short circuited, thereby shifting the resonant frequency of the security tag to a frequency which is within the detection frequency range of the EAS system, i.e., the tag is activated.
The security tag may thereafter be deactivated by exposing the resonant circuit to a sufficient level of electromagnetic energy at the new resonant frequency to short circuit the second capacitor, thereby, either preventing the resonant circuit from resonating at all or shifting the frequency of the resonant circuit to be outside of the frequency range of the EAS system, i.e., deactivating the tag. The structure and operation of an activatable/deactivatable tag of this type is described in U.S. Pat. No. 5,081,445, entitled xe2x80x9cMethod For Tagging Articles Used In Conjunction With An Electronic Article Surveillance System And Tags Or Labels In Conjunction Therewithxe2x80x9d and in U.S. Pat. No. 5,103,210, entitled xe2x80x9cActivatable/Deactivatable Security Tag For Use With An Electronic Security Systemxe2x80x9d, both of which are incorporated herein by reference.
While activatable/deactivatable security tags of the type disclosed in the above-identified patents have been shown to be very effective when utilized with EAS systems, they have been found to suffer from certain drawbacks. Security tags of this type are typically formed of a flexible, substantially planar dielectric substrate having a first conductive pattern on a first side and a second conductive pattern on a second side, the conductive patterns together establishing the resonant circuit with the substrate forming the dielectric between the plates of the capacitor(s). There is no direct electrical connection between the conductive patterns. Under certain environmental conditions, an electrostatic build-up may occur on either or both sides of the substrate. In some cases, particularly when the electrostatic charge on one side of the substrate is abruptly reduced or drained, such as when one side of the substrate is grounded to create electrostatic discharge, the voltage potential on one side of the substrate is sufficiently different from the voltage potential on the other side of the substrate to cause premature breakdown of the dielectric between the plates of one or more of the capacitors, thereby prematurely short circuiting one or more of the capacitors and either prematurely activating the security tag (in the case of the activatable/deactivatable tag) or prematurely deactivating the security tag.
One solution to the above-described electrostatic discharge problem is disclosed in U.S. Pat. No. 5,182,544, entitled xe2x80x9cSecurity Tag With Electrostatic Protectionxe2x80x9d, the subject which is hereby incorporated herein by reference. The security tag of the ""544 patent includes a static dissipation member on each side of the substrate, which effectively surrounds the two conductive patterns and temporarily maintains both sides of the substrate at substantially the same electrostatic potential during the manufacturing process. A frangible connection is provided between at least one of the conductive patterns and the surrounding static dissipation member, the frangible connection being broken when the tag is removed from its carrier for placement on an article. The breaking of the frangible connection effectively disables the electrostatic protection afforded by the static dissipation member. While the electrostatic protection methods described in U.S. Pat. No. 5,182,544 are very effective for preventing premature breakdown of the dielectric between the capacitor plates while the tag is in web form, i.e., before placement on an article, it provides no electrostatic protection once the tag is placed on an article to be protected.
A further alternative for providing electrostatic protection is taught by U.S. Pat. No. 5,754,110, entitled xe2x80x9cSecurity Tag And Manufacturing Methodxe2x80x9d the subject matter which is incorporated herein by reference. The ""110 patent teaches the concept of a discontinuous guard member which surrounds the conductive pattern on one or both sides of the substrate. However, because the guard member on the first side of the substrate is not electrically connected to the guard member on the second side of the substrate, the method disclosed in this patent is not completely effective in preventing the discharge of the electrostatic buildup which results in premature short circuiting of one of the capacitors.
The present invention comprises a security tag, which overcomes the above-described problems associate with the prior art by providing a direct electrical connection through the dielectric substrate of a tag to permanently electrically connect together a first conductive pattern on a first side of the substrate and a second conductive pattern on the second side of the substrate to thereby continuously maintain both sides of the substrate at substantially the same static charge level at all times. With a tag made in accordance with the present invention, if the electrostatic charge level on a first side of the substrate is abruptly diminished, for example, by one side of the tag being grounded, the charge level on the second side of the substrate will be likewise diminished, thereby decreasing the potential for a difference in the static charge levels on opposite side of the substrate, and thereby preventing premature short circuiting of any of the capacitors.
Briefly stated, the present invention, in one embodiment, comprises a security tag for use with an electronic security system which functions within a second frequency range. The tag comprises a substantially planar dielectric substrate having a first side and a second side. A first conductive pattern is located on the first side of the substrate, the first conductive pattern comprising at least a first inductive element, a second inductive element, a first plate of a first capacitive element and a first plate of a second capacitive element. A second conductive pattern is located on the second side of the substrate, the second conductive pattern comprising at least a second plate of the first capacitive element and a second plate of the second capacitive element, the plates of each of the capacitive elements being aligned with the inductive elements and the capacitive elements forming a resonant circuit which resonates at a first frequency within a first frequency range which is outside of the second frequency range. A direct electrical connection extends through the substrate to electrically connect the first conductive pattern to the second conductive pattern to thereby continuously maintain both sides of the substrate at substantially the same static charge level.
In a second embodiment, the present invention comprises a security tag for use with an electronic security system which functions within a second frequency range. The tag comprises a substantially planar dielectric substrate having a first side and a second side. A first conductive pattern is located on the first side of the substrate, the first conductive pattern comprising at least a first inductive element, a first plate of a first capacitive element, and a first plate of a second capacitive element. A second conductive pattern is located on the second side substrate, the second conductive pattern comprising at least a second inductive element, a second plate of the first capacitive element and a second plate of the second capacitive element with the plates of each of the capacitive elements being generally aligned. The inductive elements and the capacitive elements together form a resonant circuit which resonates at a first frequency within a first frequency range which is outside of the second frequency range. A direct electrical connections extends through the substrate to electrically connect the first conductive pattern to the second conductive pattern to thereby continuously maintain both sides of the substrate at substantially the same static charge level.
In a third embodiment, the present invention comprises a security tag for use with electronic security system which functions within a second frequency range. The tag comprises a substantially planar dielectric substrate having a first side and a second side. A first conductive pattern is located on the first side of the substrate, the first conductive pattern comprising at least a first inductive element, a second inductive element, a first plate of a first capacitive element and a first plate of a second capacitive element. A second conductive pattern is located on the second side of the substrate, the second conductive pattern comprising at least a third inductive element, a fourth inductive element, a second plate of the first capacitive element and second plate of the second capacitive element, the plates of each of the capacitive elements being generally aligned. The inductive elements and capacitive elements form a resonant circuit which resonates at a first frequency within a first frequency range which is outside of the second frequency range. A direct electrical connection extends through the substrate to electrically connect the first conductive pattern to the second conductive pattern to thereby continuously maintain both sides of the substrate at substantially the same static charge level.